Electrochemical sensors, for example, of the type described in the referenced U.S. Pat. No. 4,300,990, Maurer et al., assigned to the assignee of the present application, describes a sensor which has an electrical insulating layer between a layer-like resistance heating element and a solid electrolyte. At low operating temperatures, that is, in temperatures in the region between for example about 250.degree. to 400.degree. C., the layer-like insulation between the resistance heater and the solid electrolyte may have insufficient insulation resistance. Due to this low insulation resistance, leakage currents may arise between the heater element and the electrodes. These leakage currents tend to adulterate the sensing signals derived from the electrodes, that is, by superimposing erroneous voltages thereon. If the sensing signal is used to control the fuel-air ratio of an internal combustion engine, the control may react to this thus falsified sensing signal and may cause erroneous control of the engine. The problem does not arise in the normal working temperature range of the sensor which is above 400.degree. C., since the resistance relationship between solid electrolyte, and the insulation then will be sufficient.
The sensor can be constructed in the form of plates, as described in the referenced Maurer et al U.S. Pat. No. 4,300,990, or may be tubular, for example as described in German Patent Disclosure Document DE-OS 30 35 608. The electrodes may be located on the same side of the solid electrolyte, or on different sides. The sensors may operate in the potentiometric mode, that is, similar to a cell, or in the polarographic, or limit current mode, described, for example, in U.S. Pat. No. 4,294,679. They may operate with reference to a gas of known oxygen content, for example air, in which one electrode is exposed to the reference substance; or it may operate with both electrodes being exposed to the gas to be analyzed, in which, then, the electrodes will have different properties with respect to their electrochemical action, or characteristics, see, for example, the referenced patent 4,294,679.
The sensing elements themselves, whether plate-like or tubular, can be retained in a metal housing, and various constructions may be used; one such construction is shown, for example, in U.S. Pat. No. 4,283,261.